Renamed VCL's OutputDevice mirroring functions

I have renamed the following functions:

* ImplReMirror            -> ReMirror
* ImplHasMirroredGraphics -> HasMirroredGraphics

This has been done because these functions are definitely public
and there is no need to prefix them with "Impl" as they are exposed
to other objects.

Change-Id: Ib52493c9824e29a038de2fc35736a5ca863c091a
Reviewed-on: https://gerrit.libreoffice.org/7911
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>